1. Field of the Invention
This invention relates generally to a window assembly for transmitting light into and from a vessel containing a high pressure, high temperature, highly reactive environment. The invention relates more specifically to a window assembly which can be mounted in a pressure vessel, such as the breech of a large caliber artillery cannon, in a manner that allows the high pressure surface of the window to be substantially flush with the high pressure surface of the surrounding metal case.
2. Description of the Related Art
When employed in a weapon such as a large caliber artillery cannon, a window can facilitate the introduction of a pulse of laser light to ignite the propellant charge of the gun. In such service, however, the environment inside the gun to which the window is exposed during firing typically includes a rapid transition from ambient conditions at a pressure of one atmosphere to pressures of 400 MPa (60,000 psi) and flame temperatures near 3000 Kelvin, followed by the return to one atmosphere, in a time of about 20 milliseconds. In addition, high flow velocities of gas are present in the vicinity of the window. Because of the high pressure and gas flow rates, heat transfer rates to the gun wall, the window, and the surrounding mount in which the window is located are very high. Therefore, a window in such service must be very robust in order to survive many hundreds of gun firings without loss of either optical transmission or damage to the pressure seal.
Furthermore, the high-pressure surface of the window must be nearly flush with the surrounding mount in order to allow for rapid automatic cleaning of residue from the window between firings. The window and its seal must survive numerous gun firings without attention beyond automatic cleaning of propellant residue.
A window which employs a 20.degree. full angle conical sapphire window seated in pyrophillite and operating at pressures of up to 12 Kbars (175,000 psi) is described in Stromberg, H. D. and Schock, R. N., "A Window Configuration for High Pressure Optical Cells," The Review of Scientific Instruments, Vol. 41 (1970), pp. 1880-1881. Pyrophillite (AlSi.sub.2 O.sub.5 (OH)) is a soft, natural, mineral-like talc that is related to agalmatolite. The ductile seat distributes stress over the entire crystal sealing surface, the curved surface of the window does not need to be polished when so seated, and the compressive strength of the window is aided by placing the window material under compression. Testing of this design during the development of the present invention, however, revealed that this design is unacceptable for the performance requirements in a gun, primarily because the soft material erodes rapidly when exposed to the temperature, chemical corrosiveness, and high gas flow rates of the gun environment. Once the sealing material is eroded from the edge of the window, the window is unsupported in that region and fractures with subsequent pressure pulses.
The use of an 18.degree. full angle conical diamond window with 64 facets seated in a 0.010 inch thick gold foil pre-formed sleeve at approximately 500.degree. C. and 100 MPa (15,000 psi) is described in Marley et al., N. A., "High Temperature and Pressure System for Laser Raman
Spectroscopy of Aqueous Solutions," The Review of Scientific Instruments, Vol. 59 (1988), pp. 2247-2253. The cost of diamond windows, however, is prohibitive for production applications. When the gold seal concept was tested with sapphire windows in a gun environment, it was found to be satisfactory for short-term application. After a number of gun cycles, however, the gold was eroded by the gun gases, and the windows failed.
A method of soldering cylindrical sapphire windows into a device used at 850K and 200 MPa (30,000 psi) is described in Gorbaty, Y. E. and Bandarenko, G. V., "Soldered High-Pressure High-Temperature Sapphire Window," The Review of Scientific Instruments, Vol. 65 (1994), pp. 2739-2740. Testing of this design during the development of the present invention, however, revealed that the rapid pressure and temperature cycling to which the window is exposed in a gun application causes failure in sapphire windows mounted in accordance with this technique.
The use of sapphire cylinders sealed on the cylinder surface at conditions of up to 450.degree. C. and 360 MPa (52,000 psi) is described in Lentz, H., "A Method of Studying the Behavior of Fluid Phases at High Pressures and Temperatures," The Review of Scientific Instruments, Vol. 40 (1969), pp. 371-372. Because this design uses a deeply recessed window which is held in place by a mechanical retainer, however, it is not applicable for flush mounting.
A device employing a conical window shape and the aforementioned sealing techniques is described in U.S. Pat. No. 5,124,555 to Hartl. The disclosed device is not suitable for the present application, however, because the window is mounted substantially below the metal case surface, and the technique cannot be adapted for flush, or even near-flush, window mounting. In a gun application the recessed region on the high-pressure side of a window mounted according to this design would fill with residue after a small number of cycles and become unusable. The service life of a window with this design is also expected to be much shorter due to loading by the retaining forces on the high-pressure face of the window.
Thus, each of the aforementioned conventional designs have inadequacies which render them unsuitable for use in the service encountered by the present invention. For example, where the seal requires the window to be substantially recessed below the surface of the surrounding material or main body of the chamber, a large amount of solid residue (resulting from the gun propellant combustion) tends to accumulate on the window and in the recessed area. No reasonable rapid and automatic method of cleaning this recessed area is available once the recessed depth is below a few millimeters. An additional design weakness of windows that are recessed with a retainer ring on the high-pressure face is the uneven loading of the window that results. This additional loading may result in a much shorter service life when the window is repeatedly pressure cycled.
Second, hard seals such as soldered windows do not withstand the cyclic loading of repeated gun firings. This is because the window and mount interface must exhibit plastic behavior. Due to the thermal mismatch and the difference in material stiffness (as measured by Young's Modulus) between the steel mount (or similar high-strength, ductile material) and the sapphire window (high-strength, transparent, brittle material), movement must be allowed at the interface while intimate contact and support are maintained.
Finally, while affixing the window to the seat with an adhesive may be possible, ceramic sealants were found to be too inflexible to withstand the repeated temperature and pressure cycles encountered in a gun environment. Softer, i.e., polymeric, sealants erode from the bond area between the window and seat as a result of the high temperature, high-pressure environment, thus leaving the window partially unsupported. The unsupported portion of the window fails on subsequent cycles.